VOLUME 2, DECEMBER 2009

Mohd Akmal Mohd Yusoff

Postgraduate Student (MSc.)
B. Eng. in Mechatronic Engineering (USM)
Project Title: Design & Development of Intelligent Hybrid Underwater Vehicle for Inspection & Monitoring Application, Design & Development of Micro Underwater Platform

ROV Trial Using Power Line Communication

Remotely Operated Vehicle (ROV) has been wiedly used all around the world. Either self-made or commercially built, ROV apparently becomes more essential when dealing with underwater environment. Although there is an increase demand of AUV application, many stick with this human-operated underwater vehicle because of its efficiency when countering heavy duty and multitasking missions.

ROV is tethered based on underwater vehicle. Tether cable or often called as umbilical is the ROV lifeline support. Unlike AUV which is carrying is limited on-board power supply and preprogram missions, ROV is fed with unlimited power supply and capable of real-time data transmission. Conventional ROV communication often use twisted pair, coaxial and fiber optic cable. A new technique in ROV communication called Power Line Communication was developed by researchers from USM URRG. This technique was succesfully applied and tested on recent development of USM URRG Mini Remotely Operated Vehicle.

Why chose PLC and How It Works?

PLC works with all networking peripherals. In ROV communication, PLC introduced high speed networking, up to 200 mbps. PLC technology combines both power line and high speed networking in a single cable. PLC runs through a copper cable initially occupied by electrical voltage.

Conversion modem suppresses the digital network packet data over the power system using modulation technique. By principle, a range

of frequencies, from medium to high frequency (1.6 to 80 MHz) is superimposed at low energy level over the 50/60 Hz electrical signal. This modulated signal is transmitted over electrical medium and received by another conversion modem. The modem demodulates the signal, where high pass filter allows higher frequency signal passing through to be processed.

PLC Application Mini ROV Communication

As PLC introduced high speed networking, mini ROV communication shares the same principle. Two set computers and PLC modems are prepared, each at the surface station and inside the mini ROV. Windows XP was chosen as the operating system because of its simplicity in network configuration.

Local Area Network (LAN) setup is made on both computers by configuring the TCP/IP properties. PLC modems are plugges on computer’s Ethernet port. All modems are connected in the same physical link. Two pairs of modem are used in the mini ROV system; each pair for controller data and video transmission. The PLC system has been tested on the mini ROV through series of trial in a pool and one sea trial.

Conclusion

Overall performance for the PLC system was as good as conventional ROV communication. Instead of high speed data transmission, the advantage of using PLC is that it reduces the number of physical cable, PLC shares the same medium as power line., thus eliminate the need of another communication cable. One disadvantage of PLC technology is the distance limitation.

Originally intended for short range networking (100-200 meter), currently there are efforts made to improve the range limitation so that PLC system can be widely used in any electrical system.

Mohd Ikhwan Hadi Bin Yaakob
Postgraduate Student (PhD)
M.Sc. Physics, Instrumentations (UTM)
B.Sc. Physics, Industrial Physics (UTM)
Project Title:
Micromachined Parametric Array Acoustic Transducers for Sonar Applications

UNDERSTANDING SPECIFICATION OF UNDERWATER ACOUSTICS TRANSDUCER

Some Basic Feature

Choosing the right transducer is probably depending on your applications. Transducer for transponder will carry different specifications with transducers for echo sounder, side scan and calibrations purpose. However, transducer for pingers and acoustic telemetry might carry same basic features with the one utilized for transponder. Frequency is not the only feature need to be considered, one might also revise how deep the transducer will be submerged during operation, what type of beam pattern that might significant with the targeted application and how much power need to be generated and consumed by the sensing system. More details, the sensitivity and resolution of the expected data also urgent specifications that require more attentions during selections. All these are some basic features that need to be studied properly, whether as the end-users or the developer of this technology. There are also several other properties which are less important but still carry significant roles during operation, such as the weight of the transducer as well as the housing and cabling materials utilized. The importance of understanding all these basic features are numerous. Of course it will prevent you from spending more money for winter clothes for a short trip during summer, but what most important for us in scientific community is about the validity and reliability of the data produced from our measurements.

Frequencies of underwater acoustic transducer can be categorized into three groups of low, medium and high. The categorization of these frequencies is according to the serial resonance frequency and sometimes denoted the maximum frequency band of those transducers. Below 15 kHz of frequency, the transducer is categorized in low-frequency type and above 200 kHz of frequency is considered high-frequency transducer. In the middle of low and high resonance frequency transducers, there is medium frequency category. It is rare for underwater acousticians to apply ‘ultrasonic’ and ‘infrasonic’ terms, since the familiarity of these terms in airborne acoustic applications. Low frequency transducers usually employed for underwater positioning and navigation, acoustic telemetry, underwater telephone systems, distress pingers, transponder system, some echo sounding system as well as mine detecting equipment. High frequency transducers usually fit several applications that require extra precision data and output such as sound velocity meters, pulse echo measurements, calibration purposes, reference measurements and several echo sounding applications. For most echo sounding purpose, the frequency is in medium range as well

as the other applications such as side-scan sonar, sub-bottom profiling, distance measurements, noise measurements, audio recording, structure monitoring and marine biology related measurements as simplified in Table 1.

TABLE 1: Frequency range and applications

Another important feature is the functionality of the transducer itself; either as a hydrophone only (receiver), projector only (transmitter) or dual functionalities (transceiver). One might already choose the right operating frequency and the usable band is perfectly matched with the application but wrongly select the functionality which ending with unnecessary spending. Passive sonar system for example never require the transducer to have a transmit capability. Luckily, modern transducer design and advanced transduction mechanism material utilized in underwater acoustic sensing nowadays allow most transducers to operate as transceiver. Maximum operating depth of the transducers are also an important criteria and one of the basic features need to be considered. Maximum operating depth reflects the ability of the transducer to work efficiently under certain amount of hydrostatic pressure. Beyond this limit, transducer may either been damaged by maximum diaphragm deflection or sustaining collapse voltage inside electro-acoustic transduction medium. However, most modern transducer might not suffer any damage even being accidentally operated beyond the maximum operating depth. There is another parameter called survival depth that provides spare life span for the transducer. The value of survival depth usually set around 10% more than the maximum operating depth allowed. Temperature is also an important parameter that varies with depth. Some electro-acoustic materials may produce unsystematic error due to temperature variation. Usually, manufacturer will suggest two temperature limitations which are operating and storage temperature ranges. Fulfilling operating temperature range will ensure the reliability and validity of the measured data. Storage temperature range on the other hand, functions more as a precaution action when keeping the transducer to ensure its safety. Storage casing provided by some manufacturer may contain heat insulation element to protect the device from extreme heat and cold, so the usage of such provided accessory is always mandatory.

Important Properties

Acoustic is a science of sound. Sound is a pressure disturbance that occurs inside a medium. Sound is measured as pressure (N/m2) or Pascal and sound pressure level (SPL) is measured using an absolute unit called decibel and denoted with dB. Decibel unit requires reference value, thus reference sound pressure for underwater was set at 1 µPa and for airborne applications, the reference sound pressure is 20 µPa.

These standard references are defined in ANSI S1.1-1994 documents. Input sensitivity which also known as receiving response is measured in dB reference to 1V/µPa. This frequency function represent how much voltage will be produced by the transducer correspond to every µPa of sound pressure received at the surface of the transducer. However, maximum or peak value of receiving response provided in the datasheet only occurs at resonance frequency of the transducer. A flat response which is slightly lower than peak can be expected within the side lobes and one of the important measures to determine the quality of the hydrophone. It is very important for the operator to have an approximate value of output voltage in correspond to every measurement so that the right gain can be set using amplifier for perfect resolution of data presentations.

For projectors, one of the important properties is transmitting response. Just like a receiving response, it is a frequency function. However, transmitting response represent how much µPa of sound pressure generated in correspond to 1V of drive voltage, 1 meter away from the transducer. Some manufacturers used sound intensity level (SIL) instead of SPL in presenting transmitting response. The unit for transmitting response is dB re µPa/1V @ 1m. Acoustic projectors also having their maximum output capacities at resonance frequency. 1V of AC drive voltage at resonance frequency might produce 150 dB of SIL, but at other frequencies the SIL might be lower even at the same amplitude of drive voltage. Projectors usually operated at its resonance frequency. A flat response region is not really important for many applications that require single frequency transmission. For tunable and multiple frequencies applications, broad frequency band is essential so that any signal at desired frequency can be generated with enough intensity. Another important characteristic for underwater acoustic transducers is directionality or beam pattern. It measures the SIL or SPL as a function of operating angles. The pattern usually represented in horizontal and vertical planes at a specific given frequency. At slightly different frequencies however, user can still consider the same pattern. Spherical transducer usually produced flat and same pattern over all angles as well as vertical and horizontal planes. But there are other shape of transducer such as toroidal and flat. Beam pattern become important for target specific applications such as side-scanning and profiling where the signal beam need to be transmitted to a specific area and the detection is also made from specific pre-determine angle only. This will help a lot in reducing the noise from any unwanted sources from random location. For other applications such as underwater surveillance and noise measurements, directionality and beam pattern might be less important.

References

[1] C.H. Sherman and J.L. Butler, “Transducers and Arrays for Underwater Sound,” Springer, pp. 321-324, 2007.

[2] S. Aravamudhan and S. Bhansali, “Reinforced piezoresistive pressure sensor for ocean depth measurements,” Sensors and Actuators A; Physical, vol. 142, pp. 111-117, 2008.

[3] M.E. Motamedi, “Acoustic Sensor Technology”, IEEE MTT-S Digest, pp. 521-524, 1994.

VOLUME 2, NOVEMBER 2009

Mohd Ikhwan Hadi Bin Yaakob

Postgraduate Student (PhD)
M.Sc. Physics, Instrumentations (UTM)
B.Sc. Physics, Industrial Physics (UTM)
Project Title:
Micromachined Parametric Array Acoustic Transducers for Sonar Applications

Underwater Acoustic Microsensors : Design and Apllication

Abstract

A wide variety of microsensors are currently being fabricated by micromachining technology. A micromachined sensor is not only much smaller than conventional one, but also offers several other advantages. From the first design reported two decades ago until the most recent, the development of acoustic microsensors for underwater applications has grown in several aspects. This multidisciplinary research area has attracted many researchers from different niche areas. As a result, enorrmous designs available today, developed using multiple techniques by utilizing various materials. This poster will discuss very brief facts on advantages, designs, specifications and applications of this sensor.

Why Micro?

1. Permit electronics to be fabricated on the same silicon chip as transducer
2. Capability to fabricate arrays with a large number of elements per unit area
3. Capability to have more built-in intelligence (perform self-diagnostic and self-calibration)
4. Silicon technology – Mass production at low cost

Application & Specifications

VOLUME 2, NOVEMBER 2009 

Mohd Ikhwan Hadi Bin Yaakob

Postgraduate Student (PhD)
M.Sc. Physics, Instrumentations (UTM)
B.Sc. Physics, Industrial Physics (UTM)
Project Title:
Micromachined Parametric Array Acoustic Transducers for Sonar Applications

Urgent Needs

Generally, when a sensor or transducer become smaller, sensing area will also shrunk and the amount of sample substances required to produce electrical output will be minimal. The consequences might be good and might also be bad. The former is making sense when sensitivity and resolution of the transducer are taken into account. The latter on the other hand, start taking place when operating environment is noisy. In terms of dimension, the advantage depends on the applications. Underwater platforms exist in various sizes and dimensions. Each platform carries specific number of task and functions.

Power capacity for every platform would be differ as well. For instance, utilizing the array of kilowatts tonpilz acoustics transducers with several meters in diameter inside the Akula class submarine has never been an issue, since the capacity of the platform meets performance requirements of the sensing unit. In contrast, compact platforms such as torpedo, ROV, AUV, underwater glider and surface vessel need compact and robust acoustic sensing mechanism in order to meet desired sounding resolution with limited spaces and power capability. In terms of intensity or SPL, it is odd to compare between conventional transducer designs and MEMS based transducers. However as stated earlier, comparison of sensitivity, directivity and resolution might be even.

Advantages of MEMS

In general, advantages of MEMS compare to macro devices and systems (not all of them) come in many different ways other than sizes. By adopting the same fabrication technologies with microelectronics and IC, total development cost of micro scale devices are significantly reduced. Cheap silicon based materials also contribute to the effectiveness of the development and fabrication cost. Developed transducer is possibly match to be integrated with any support electronics within the same chip. The integration will at least reducing the parasitic capacitive noise level on the systems.

Bandwidth of the transducer determines the range of usable frequencies of acoustics signal that can be detected and projected. For sonar applications, wider bandwidth might possibly place a single transducer over several applications. In underwater communications, broad range of operating frequencies will increase the data transfer rate thus allowing fast and clear underwater communication channel. Another issue on conventional design is impedance mismatch. Without a matching material, maximum power transfer from the transducer to the load (as well as from the load to the transducer in hydrophone) will be impossible. Two pre-requisites properties for the matching layer are acoustic impedance of the material and the thickness of the matching layer itself. The simplest calculation for matching layer thickness is quarter of the wavelength of the signals for maximum power transfer. For high frequency operations that employ shorter pulses of signal, quarter wavelength will be less than 1 mm thick. Using standard machining and cutting, it is impractical to fabricate such layer thickness. In case of complex array, wiring every element in the array is one of the most tedious jobs.

MEMS based acoustic sensor however, able to operate in relatively wider frequency band and resonance frequency tuning can be controlled easily without fully depending on the thickness of piezoactive material. Previous study [1] found that the resonance frequency of MEMS b

ased acoustic device with vibrating diaphragm and membrane can be controlled by varying the overall thickness of the diaphragm. A few years later [2], the width of the membrane was proved to contribute more in shifting not only the resonance frequency, but the acoustic impedance and the coupling coefficient of the transducer. These features give a MEMS based transducer more design flexibility tailing by various underwater applications. By adjusting the width of the diaphragm while keeping the thickness value constant, acoustic impedance of the device can be shifted to be near the acoustic impedance of the load. For underwater applications, approximate acoustic impedance of sea water is around 1.5 MRayl.

Matched impedance ensures maximum energy transfer and increase the total efficiency of the sensor system, made this type of transducer suitable for platforms with limited power resources.  From the perspective of fabrications, MEMS technology allows two dimensions array of identical elements being fabricated on the thin layer of silicon wafer. Every element is wired with internal electrodes with no hard wiring needed.

Available Design of Acoustic Microsensors

Piezoelectric usually found in ferroelectric materials with high piezoelectric constant and strong electromechanical coupling factors. PZT are most popular material which sustain superior dielectric constant and thermally stable. Piezoelectric layer in the diaphragm is poled in the thickness direction by applying electrical field. With the present of electrical field across the thickness, strain induces the diaphragm to bend and causing the pressure disturbance with the medium in contact with the transducer. During flexural motion of the diaphragm, charge displacement took place in electroded piezoelectric layer and will be detected by receiving circuitry.

Capacitive type transducer consists of miniaturized parallel plate capacitor. The dimensions of the plate is around 10’s of micrometer with the gap between plates in several 10’s to 100’s of nanometer. Capacitance value for this type of transducer usually in the order of picoFarad.

The back plate position is fixed and the top plate is moving or vibrating under the effect of electrostatic attraction between oppositely charged plate and the opposing restoring force provided by the stiffness of the flexing diaphragm. Basic performance parameter such as bandwidth, dynamic range and electromechanical coupling coefficient was found parallel with the conventional design.  The working philosophy is as simple as the change in capacitance value when a top plate vibrating in corresponds to the received acoustic pressure [3].

 

There are several limitations exist in capacitive type transducer design. One of those is voltage bias that needs to be apply in achieving desired coupling coefficient usually near exceeding collapse voltage. Using current existing fabrication technology, it is difficult to secure intended safety margin of the bias voltage to avoid collapse in all array element. Piezoelectric type however, does not require any bias voltage.

Capacitive type transducer also need different design for receive and transmit operation. In maximizing the sensitivity, receiving element need to have a thin gap and transmit element need to has large gap to allow large plate deflection. Compare to piezoelectric type transducer, capacitive types was proven to have better sensitivity.

References
[1]           Ko, S.C., Kim, Y.C., Lee, S.S., Choi, S.H. and Kim, S.R. (2003) Micromachined piezoelectric membrane acoustic device. Sensors and Actuators A (103), pp130-134.
[2]          Akasheh, F, Myers, T., Fraser, J.D., Bose, S. and Bandyopadhyay, A. (2004) Development of Piezoelectric Micromachined Ultrasonic Transducers. Sensors and Actuators A (111) pp 275-287.
[3]           Cianci, E., Schina, A., Minotti, A., Quaresima, S. and Foglietti, V. (2006) Dual frequency PECVD silicon nitride for fabrication of CMUTs’ membranes. Sensors and Actuators A (127), pp 80-87.